Sorry to hear that you're struggling at such an early stage. To address your issues in reverse order -
Yes, flat lacquer spray (Dulcote) will remove the gloss from decals.
As to your paint not adhering - the details you provided would seem to rule out two of the most likely causes, but I will still ask: you say you cleaned the hull first. Was that with a plastic prep solution or rubbing alcohol? No chance of any residue of any kind?
You say you also tried various types of tape. That would seem to rule out the wrong kind of tape.
I have no experience at all with acrylics, but I've always been skeptical of their ability to bond to styrene plastic - truly bond, as opposed to simply coating. Paint thinner for oil-based paints will "melt" the plastic if applied directly, and oil-based paints, being thinned heavily, will therefore, to my mind, bond directly to the plastic. My tape gripped the painted area of the hull pretty tightly (I am talking about 3/4" wide blue painter's tape) and I had to peel it up with some force, yet no paint came up. I did not have to touch up a single spot. I have to wonder how it would be possible for acrylic paint to bond to the plastic as well, considering water (the base for acrylics) will not act as a solvent for plastic in the least.
I am neither a chemist nor a paint expert (except for applying it to a Titanic model with a fair amount of success.) I think you need someone who's experienced with acrylics to offer their thoughts.